The present invention generally relates to cam shafts of combustible engines and the like. More particularly, the present invention relates to an adjustable cam shaft which enables intake and exhaust timing to be altered without the need to replace the cam shaft.
The cam shaft itself is an age-old mechanical device that converts rotary motion to linear motion. The cam shaft opens and closes intake and exhaust valves to let the air/fuel mixture into the cylinder and the exhaust out. The cam shaft includes lobes which lift the valves. The taller and the greater the diameter of the lobes, the higher and longer it will lift the valves, allowing more air/fuel into the engine and more exhaust out, both of which should make the engine run better. The height of the lobe, or the distance it opens the valve, is known as lift and is given in thousandths of an inch. The width or fatness of the lobe determines the amount of time (relative to the crank shaft cycle) it will keep the valve open and is known as duration and is given in degrees of crank shaft rotation.
However, optimizing the performance of the engine is not so simple and many variables, including timing, must be taken into account. The amount of time, the intake and exhaust valves are closed, thus sealing the cylinder, determines how long pressure pushes on the piston. The point at which the intake valve closes and the point at which the exhaust valve opens are both critical to making power in the engine. These are just two of the timing events that can be altered on a cam shaft. When the intake valve opens, time is required to get the column of air in the ports to start flowing through it. Although the exhaust has a similar problem, it is to a much lesser degree because the cylinder is pressurized when the exhaust valve opens.
While the RPM of the engine increases, the “lag time” of the intake charge tends to remain the same and its momentum will keep it flowing into the cylinder even at the end of the intake stroke if the intake valve is kept open a little longer. Therefore, as an engine runs faster and faster, the cam shaft timing should occur later and later to keep pace with the air/fuel charge, which gets moving slower and keeps moving longer relative to engine speed.
There are three basic ways to alter cam shaft timing or tuning: lobe profile, lobe separation and the cam shaft installation position. In the past, the first two have been determined when the cam is manufactured and have not been able to be adjusted or varied without regrinding the cam shaft. Thus, in the past, a manufacturer or engine owner had to decide which operating range he or she wanted to optimize in the engine and select a cam shaft that would optimize air flow and cylinder pressure in that range. The problem exists that once the cam shaft is selected and installed, it optimizes one driving range, not others.
A cam shaft is typically installed in the engine relative to the crank/piston position and if one “lines up the dots” on the crank and cam timing gears, the number one cylinder is at top dead center, and the cam will be positioned near the middle of overlap for that cylinder. If the cam shaft is moved ahead in relation to the crank/position timed piston timing, it has been advanced, and if it is moved back, it is referred to as being retarded.
The lobe profile is the size and shape of the cam lobe, which determines how high and how long the valve opens, and is determined when the cam is manufactured. The lobe separation angle, also known as the lobe center line displacement angle, is the angle between the center line of the intake lobe and the center line of the exhaust lobe for one cylinder, as measured on the cam. That is, the timing of the intake lobe relative to the exhaust lobe is determined by the placement of the lobes on the cam shaft. The standard lobe separation angle is 110°, and in the past has been determined upon manufacturing of the cam shaft. When a cam shaft is manufactured, it can be ground with a “narrower”, e.g. 108° or 106°, or “wider”, 112° or 114° lobe separation. In the past, once these have been ground, they have been set and unalterable. The relative camshaft to crankshaft timing is dependent in regards to narrowing or widening of the lobe separation angles which directly affects the intake and or exhaust valve to open or close sooner. Widening the lobe centerline displacement angle provides increased torque, maintains idle quality, helps lowers emissions and increases vacuum for accessories such as power brakes. For racing applications a decreased lobe separation angle and advanced cam timing increases higher RPMs and horsepower numbers.
It would be desirable to alter the lobe separation displacement angle depending upon the intended performance use of the engine. For example, if the engine were to be used in a tow vehicle or motor home application, increased torque would be desirable. However in a race car setting, idle quality and low vacuum response may not be as important as high RPMs would come into play and, thus the lobe separation value would be ground closer. Accordingly there is a continuing need for an adjustable cam shaft which enables valve timing, and particularly the lobe separation angle, to be selectively adjusted. The present invention fulfills these needs and provides other related advantages.